A Bcl6 Intronic Element Regulates T Follicular Helper Cell Differentiation

In response to an intracellular infectious agent, the immune system produces a specific cellular response as well as a T cell–dependent Ab response. Precursor T cells differentiate into effector T cells, including Th1 cells, and T follicular helper (TFH) cells. The latter cooperate with B cells to form germinal centers and induce the formation of Ab-forming plasmacytes. One major focal point for control of T cell differentiation is the transcription factor BCL6. In this study, we demonstrated that the Bcl6 gene is regulated by FOXO1-binding, cis-acting sequences located in a highly conserved region of the first Bcl6 intron. In both mouse and human T cells, deletion of the tandem FOXO1 binding sites increased the expression of BCL6 and enhanced the proportion of TFH cells. These results reveal a fundamental control point for cellular versus humoral immunity.

Bromodomain protein BRD4 directs and sustains CD8 T cell differentiation during infection

In response to infection, pathogen-specific CD8 T cells differentiate into functionally diverse effector and memory T cell populations critical for resolving disease and providing durable immunity. Through small-molecule inhibition, RNAi studies, and induced genetic deletion, we reveal an essential role for the chromatin modifier and BET family member BRD4 in supporting the differentiation and maintenance of terminally fated effector CD8 T cells during infection. BRD4 bound diverse regulatory regions critical to effector T cell differentiation and controlled transcriptional activity of terminal effector-specific super-enhancers in vivo. Consequentially, induced deletion of Brd4 or small molecule-mediated BET inhibition impaired maintenance of a terminal effector T cell phenotype. BRD4 was also required for terminal differentiation of CD8 T cells in the tumor microenvironment in murine models, which we show has implications for immunotherapies. Taken together, these data reveal an unappreciated requirement for BRD4 in coordinating activity of cis regulatory elements to control CD8 T cell fate and lineage stability.

Bromodomain protein BRD4 directs CD8 T cell differentiation during infection and cancer

In response to infection and malignancy, antigen-specific CD8 T cells differentiate into functionally diverse populations that are critical for resolving disease and providing durable immunity. Here, we utilized an in vivo RNAi screening strategy to identify transcriptional and epigenetic regulators controlling CD8 T cell fate. Through small-molecule inhibition, RNAi studies, and induced genetic deletion, we revealed an essential role for the chromatin modifier and BET family member BRD4 in supporting T cell differentiation during infection and cancer. BRD4 was found to bind diverse regulatory regions critical to effector T cell differentiation and was essential for transcriptional activity of terminal effector-specific super-enhancers in vivo. Consequentially, induced deletion or chemical inhibition of BRD4 resulted in impaired maintenance of a terminal effector phenotype. In preclinical models of cancer, BRD4 was also required for formation of a terminally differentiated CD8 T cell state in the tumor microenvironment, which we show holds implications for immunotherapies. Tailored inhibition of BRD4 enhanced immune checkpoint blockade efficacy; however, complete ablation of BRD4 activity blunted T cell effector function and adoptive cell therapy efficacy. Taken together, these data reveal an unappreciated requirement for BRD4 in coordinating activity of cis-regulatory elements to control CD8 T cell fate and stability of lineage identity, and provide insight for understanding the efficacy and full therapeutic potential of BET inhibition and T cell immunotherapies.

FOXO1 constrains activation and regulates senescence in CD8 T cells

Naive and memory T cells are maintained in a quiescent state, yet capable of rapid response and differentiation to antigen challenge via molecular mechanisms that are not fully understood. In naive cells, the deletion of Foxo1 following thymic development results in the increased expression of multiple AP-1 family members, rendering T cells less able to respond to antigenic challenge. Similarly, in the absence of FOXO1, post-infection memory T cells exhibit the characteristics of extended activation and senescence. Age-based analysis of human peripheral T cells reveals that levels of FOXO1 and its downstream target, TCF7, are inversely related to host age, whereas the opposite is found for AP-1 factors. These characteristics of aging also correlate with the formation of T cells manifesting features of cellular senescence. Our work illustrates a role for FOXO1 in the active maintenance of stem-like properties in T cells at the timescales of acute infection and organismal life span.

Epigenetic targeting of Brd4 constrains CD8+ T cell differentiation during infection and cancer

Memory CD8+ T cells afford long-lived, durable protection against infection and malignancy. Despite the critical relevance of memory T cells to vaccination and immunotherapies, the transcriptional and epigenetic signals instructing memory T cell fate remain unclear. Here, we utilized a pooled RNA interference screen to evaluate the activity of transcription factors and chromatin modifiers governing memory T cell differentiation in vivo. The epigenetic ‘reader,’ Brd4 of the BET protein family, emerged as a top regulator of memory T cell differentiation. Through RNA interference, small molecule inhibition and inducible genetic deletion, we established a central role for Brd4 in mediating CD8+ T cell proliferation, differentiation and function during acute viral infection. Brd4 was required for optimal expression of fate-specifying transcription factors, and a deficiency of Brd4 activity resulted in diminished formation of effector memory and CD103+ tissue-resident memory CD8+ T cells. Given that BET inhibition has emerged as a powerful approach for suppressing tumor growth, we also evaluated how BET inhibition and Brd4-deficiency influence T cell activity during cancer. Brd4 was required for intratumoral accumulation and function of T cells in a mouse model of melanoma, and BET inhibition suppressed T cell-mediated control of tumor growth. However, epigenetic targeting of Brd4 constrained terminal differentiation of T cells within tumors, biasing tumor-resident T cells towards a reprogrammable TCF1-expressing phenotype. These studies establish a novel role for Brd4 in CD8+ T cell biology and provide insight for immunotherapy approaches designed to leverage the dynamic activity of CD8+ T cells in tumors.

Profiling the lymphoid-resident T cell pool reveals modulation by age and microbiota

Despite being implicated in non-lymphoid tissues, non-recirculating T cells may also exist in secondary lymphoid organs (SLO). However, a detailed characterization of this lymphoid-resident T cell pool has not yet been done. Here we show that a substantial proportion of CD4 regulatory (Treg) and memory (Tmem) cells establish long-term residence in the SLOs of specific pathogen-free mice. Of these SLOs, only T cell residence within Peyer's patches is affected by microbiota. Resident CD4 Treg and CD4 Tmem cells from lymph nodes and non-lymphoid tissues share many phenotypic and functional characteristics. The percentage of resident T cells in SLOs increases considerably with age, with S1PR1 downregulation possibly contributing to this altered homeostasis. Our results thus show that T cell residence is not only a hallmark of non-lymphoid tissues, but can be extended to secondary lymphoid organs.

Continuous activity of Foxo1 is required to prevent anergy and maintain the memory state of CD8+ T cells

Delpoux et al. show, in a model of latent infection, how FOXO1 is required to prevent apoptosis, the acquisition of an anergy phenotype, and to be constantly expressed for maintaining the differentiation state of CD8⁺ T cells.

Upon infection with an intracellular pathogen, cytotoxic CD8⁺ T cells develop diverse differentiation states characterized by function, localization, longevity, and the capacity for self-renewal. The program of differentiation is determined, in part, by FOXO1, a transcription factor known to integrate extrinsic input in order to specify survival, DNA repair, self-renewal, and proliferation. At issue is whether the state of T cell differentiation is specified by initial conditions of activation or is actively maintained. To study the spectrum of T cell differentiation, we have analyzed an infection with mouse cytomegalovirus, a persistent-latent virus that elicits different cytotoxic T cell responses characterized as acute resolving or inflationary. Our results show that FOXO1 is continuously required for all the phenotypic characteristics of memory-effector T cells such that with acute inactivation of the gene encoding FOXO1, T cells revert to a short-lived effector phenotype, exhibit reduced viability, and manifest characteristics of anergy.

Crucial role of FOXO1 in inflationary CD8+ T-cell responses during persistent viral infection

Cytomegalovirus (CMV) establish persistent infection that induce the accumulation of virus-specific T-cells over time in a process called memory inflation. Although persistence of antigen (Ag) is considered essential, the molecular pathway for the inflationary T-cell development is poorly understood. Mainly, inflationary CD8 T-cells are KLRG1hi CD27−, but a low percentage of them are KLRG1lo CD27+, which are considered as their memory precursors. To study the role of FOXO1 in CD8 T cells during MCMV infection, we analyzed mouse bone marrow chimeras in which FOXO1 was specifically deleted in CD8 T cells. These mice were infected with MCMV-Δm157 strain, and the expansion and phenotype of inflationary was examined. The results revealed that the IE3 and M38 CD8 T-cell responses do not inflate in absence of FOXO1. Furthermore, with an absence of FOXO1, there were fewer Ag-specific CD8 T cells that produced both IFNγ and TNFα. Consistent with this lack of CD8 effector cells the mice were less able to control the virus in the spleen and the liver at day 6 after infection. Moreover, the FOXO1 KO Ag-specific CD8 T cells fail to up-regulate the memory associated transcription factors, TCF-1 and EOMES, correlating with a lower percentage of memory precursors. Then, we found that FOXO1 KO Ag-specific CD8 T cells display an anergic state and are prone to apoptosis. Finally, we delete FOXO1 at day 40 after infection, we showed that the FOXO1 KO Ag-specific CD8 T cells survive less efficiently and lose their memory characteristics as well as their recall capacities. Collectively, these results demonstrate an intrinsic role for FOXO1 in establishing and maintaining the inflationary memory program that is essential to forming long-lived effector and memory cells.

Gain of Toxicity from ALS/FTD-Linked Repeat Expansions in C9ORF72 Is Alleviated by Antisense Oligonucleotides Targeting GGGGCC-Containing RNAs

Hexanucleotide expansions in C9ORF72 are the most frequent genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Disease mechanisms were evaluated in mice expressing C9ORF72 RNAs with up to 450 GGGGCC repeats or with one or both C9orf72 alleles inactivated. Chronic 50% reduction of C9ORF72 did not provoke disease, while its absence produced splenomegaly, enlarged lymph nodes, and mild social interaction deficits, but not motor dysfunction. Hexanucleotide expansions caused age-, repeat-length-, and expression-level-dependent accumulation of RNA foci and dipeptide-repeat proteins synthesized by AUG-independent translation, accompanied by loss of hippocampal neurons, increased anxiety, and impaired cognitive function. Single-dose injection of antisense oligonucleotides (ASOs) that target repeat-containing RNAs but preserve levels of mRNAs encoding C9ORF72 produced sustained reductions in RNA foci and dipeptide-repeat proteins, and ameliorated behavioral deficits. These efforts identify gain of toxicity as a central disease mechanism caused by repeat-expanded C9ORF72 and establish the feasibility of ASO-mediated therapy.

Crucial role of FOXO1 in inflationary CD8+ T-cell responses during persistent viral infection

Cytomegalovirus (CMV) establish persistent infection that induce the accumulation of virus-specific T cells over time in a process called memory inflation. Although persistence of antigen (Ag) is considered essential, the molecular pathway for the inflationary T-cell development is poorly understood. Mainly, inflationary CD8 T-cells have an effector-memory phenotype (KLRG1hi CD27−), but a low percentage of them express a central-memory phenotype (KLRG1lo CD27+), which are considered as their memory precursors. To study the role of FOXO1 in CD8 T cells during a persistent viral infection, we analyzed mouse bone marrow chimeras in which FOXO1 was specifically deleted in CD8 T cells. These mice were infected with MCMV-Δm157 strain, and the expansion and phenotype of inflationary and acute-contracting T cells was examined. The results revealed that the IE3 and M38 CD8 T cell responses do not inflate in absence of FOXO1. Furthermore, with an absence of FOXO1, there were fewer Ag-specific CD8 T cells that produced both IFNγ and TNFα. Consistent with this lack of CD8 effector cells the mice were less able to control the virus in the spleen and the liver at day 6 after infection. Moreover, we found that FOXO1 KO Ag-specific CD8 T cells fail to up-regulate the memory associated transcription factors, TCF-1 and EOMES, correlating with a lower percentage of memory precursors (KLRG1lo CD27+). Finally, we found that FOXO1 KO Ag-specific CD8 T cells cycle normally (Ki67+), but display an anergic state, as measured by calcium mobilization activation, and are prone to apoptosis. Collectively, these results demonstrate an intrinsic role for FOXO1 in establishing the inflationary memory program that is essential to forming long-lived effector memory cells

TCR signaling events are required for maintaining CD4 regulatory T cell numbers and suppressive capacities in the periphery

CD4 regulatory T cells (Tregs) can be subdivided into two subsets according to Ly-6C expression in the periphery. Phenotypic analysis, imaging, and adoptive-transfer experiments of peripheral Ly-6C(-) and Ly-6C(+) Tregs reveal that the nonexpression of Ly-6C by ∼70% of peripheral Tregs depends on TCR signaling events. Interestingly, Ly-6C(-) Tregs express higher surface amounts of key immunosuppressive molecules than do Ly-6C(+) Tregs and produce constitutively anti-inflammatory cytokines. In line with their phenotype, Ly-6C(+) Tregs exhibit poor suppressive capacities in vitro and in vivo. Finally, although Ly-6C(-) Tregs maintain their numbers with age, Ly-6C(+) Tregs gradually disappear. Altogether, our data strongly suggest that both the survival and suppressive functions of peripheral CD4 Tregs rely on their ability to receive strong TCR signals.